Phosphate esters have utility as basestocks in functional fluid applications such as lubricants and hydraulic fluids. These ester basestocks are particularly desirable where fire retardance and high temperature stability characteristics are required.
Deterioration of phosphate esters is evidenced by increasing acidity, presumably from hydrolytic/oxidative degradation. Eventually the acidity of the phosphate ester will increase to the point where it is corrosive for its intended use.
It is known to use chemical additives to enhance the resistance of phosphate esters to acid formation. U.S. Pat. Nos. 3,718,596, 3,723,320, and 3,649,721 describe the use of epoxides for phosphate ester stabilization. U.S. Pat. No. 4,169,800 recites the use of benzotriazole as a corrosion inhibitor. U.S. Pat. No. 3,071,549 recites various amines as oxidation inhibitors.
A number of different compounds have been used to control the acid formation in phosphate ester functional fluids. However, for each acid-control compound in use there are disadvantages such as short effective life or deleterious byproduct formation.
It is desirable to develop acid-resistant additives for phosphate esters which are effective at low concentrations, have long life under adverse conditions, permit only gradual acid buildup, and do not form unwanted byproducts.